1. Field of the Invention
The present invention relates to a semiconductor device including a circuit fabricated from a thin film transistor (hereinafter, referred to as a TFT) and a manufacturing method thereof. For example, the invention relates to an electronic device equipped with a light emitting display device having an organic light emitting element as a component.
A semiconductor device in this specification refers to a general device which can be operated by using semiconductor properties, and an electro-optic device, a semiconductor circuit, and a electronic device are all referred as a semiconductor device.
2. Description of the Related Art
In recent years, research related to a light emitting device having an EL element as a self-luminous light emitting element has been advanced. The light emitting device is referred to as an organic EL display or an organic light emitting diode. Since these light emitting devices have characteristics such as fast response suitable for movie display, low voltage operation, low power consumption, or the like, they attracts much attention as a next generation display including new generation's cellular phones and personal digital assistants (PDA).
An EL element having a layer containing an organic compound layer as a light emitting layer has a structure in which a layer containing a layer containing an organic compound layer (hereinafter, referred to as an EL layer) is sandwiched between an anode and a cathode. Electroluminescence is generated in the EL layer by applying an electronic field to the anode and the cathode. Luminescence can be obtained from the EL element when a singlet excited state returns back to a ground state (fluorescence) or when a triplet excited state returns to a ground state (phosphorescence).
The EL layer has a layered structure typified by “a hole transporting layer, a light emitting layer and an electron transporting layer”. EL materials for forming an EL layer are classified broadly into low-molecular weight (monomer) materials and high-molecular weight (polymer) materials. The low-molecular weight materials are deposited using a vapor deposition (evaporation) system.
A conventional vapor deposition system has a substrate holder where a substrate is set, a crucible (an evaporation boat) encapsulating an EL material, that is a deposition material, a shutter for preventing the EL material to be sublimed from rising, and a heater for heating the EL material in a container. Then, the EL material heated by the heater is sublimed and deposited on a rotating substrate. At this time, the distance between the substrate and the crucible needs to be 1 m or more so as to perform uniform deposition.
As to a conventional vapor deposition system and a conventional vapor deposition method, when an EL layer is formed by vapor deposition, almost all the sublimed EL material is adhered to an inner wall of a film formation chamber of the vapor deposition system, a shutter or an adherence preventive shield (a protective plate for preventing a deposition material from adhering to an inner wall of the film formation chamber) inside of the film formation chamber. Therefore, in forming the EL layer, an efficiency of utilizing the expensive EL materials is extremely low i.e. about 1% or less, and thus, manufacturing costs of a light emitting device has been very high.
As to a conventional vapor deposition system, in order to provide a film with uniform thickness, it is necessary to separate a substrate from a source of the vapor deposition 1 m or more. Therefore, the vapor deposition system has grown in size, a period required for exhausting each film formation chamber of the vapor deposition system has been prolonged; thus, film formation speed is slowed down and throughput is reduced. Also, in using a large substrate, the film thickness between a center portion and a marginal portion of a substrate easily becomes uneven. Further, the vapor deposition system has a structure for rotating a substrate; therefore, there is a limit in a vapor deposition system for a large substrate.
In view of the above-described problems, the present inventors have proposed a vapor deposition system (Reference 1: Japanese Patent Laid-Open No. 2001-247959 and Reference 2: Japanese Patent Laid-Open No. 2002-60926).